The electrophoretic effect is well known and many display devices have been designed using the electrophoretic effect to produce graphic images. One type of conventional electrophoretic display panel is shown in U.S. Pat. Nos. 4,655,897 and 4,742,345, which are commonly owned by the assignee of the present application. The electrophoretic display panel has grid and cathode conductors spaced from an anode conductor with an electrophoretic dispersion in between them. Particles of a dieletric pigment material having a light color are uniformly dispersed in a dark-colored non-conductive suspension medium. The particles in different pixel areas of the display can be made to migrate towards the anode by selectively biasing the cathode negatively with respect to the anode. The migration of the particles from the cathode to the anode, or vice versa, is used to form an image by a change in contrast of the light-colored particles against a dark-colored background of the medium.
An electrophoretic display of the above-described type has many advantages in that the materials used are relatively inexpensive, while the image formed can be maintained even when the power is removed. In order to erase the image, the cathode is biased positively with respect to the anode, i.e. to create an electric field of the opposite polarity.
In the prior art electrophoretic display devices, the anode is a unitary planar structure to which one voltage is applied in the write mode and a different voltage is applied in the erase mode. All lines of the displayed image are erased simultaneously upon application of the erase voltage to the anode, and all lines of the display must be rewritten to form the next image frame. The next frame may often have character lines or image portions which are the same as the previous frame. Because all lines are rewritten each time a new frame is displayed, the process of displaying a new frame is slowed accordingly.
It is therefore an object of the invention to provide an electrophoretic display which overcomes the aforementioned disadvantage of conventional devices. In particular, the object of the invention is to provide an electrophoretic display in which one or more lines of the display can be selectively erased and rewritten without disturbing the other image lines which remain the same from one frame to the next. It is a further object to provide a simple and inexpensive circuitry for enabling such selective line erasure in an electrophoretic display.